3D active demagnetization of cold magnetic shields

Passive shielding and active canceling of magnetic fields have their own advantages and disadvantages. To get the best of both worlds we combine both techniques. In particular, we actively demagnetize a set of original mu-metal magnetic shields instead of applying a compensating field to the shielded "payload". As a result, the new system is free of bulky Helmholtz coils. It provides reasonable shielding even when the active Flux Locked Loop (FLL) electronics is turned off. An original single-chip 3-axis Superconductor Quantum Interference Filter (SQIF) based magnetic sensor monitors the residual magnetic field with very high sensitivity, and an optional activation of FLL electronics additionally depresses low (up to 1 kHz) frequency magnetic noise including its DC component by at least two orders of magnitude. This or a similar setup could be a useful addition for experiments with any superconducting circuits that might be affected by trapped magnetic flux.